1. Field of the Invention
The present invention relates to a Group III nitride semiconductor light-emitting device. More particularly, the present invention relates to a Group III nitride semiconductor light-emitting device in which the stress applied to a light-emitting layer is relaxed.
2. Background Art
In recent years, there have been put into practice light-emitting devices employing a Group III nitride semiconductor (e.g., GaN), including high-brightness blue LEDs. Generally, a light-emitting device includes an n-electrode, an n-type cladding layer, a light-emitting layer, a p-type cladding layer, and a p-electrode, and these layers have different lattice constants.
In the case where, for example, an upper layer is formed on a lower layer through epitaxial crystal growth for producing a semiconductor light-emitting device, when the lattice constant of the upper layer greatly differs from that of the lower layer, difficulty is encountered in growing the crystal forming the upper layer. In many cases, the thus-grown crystal exhibits poor crystallinity. In addition, stress is applied between the lower layer and the upper layer even after production of the semiconductor light-emitting device. A piezoelectric field is generated in the layer to which stress is applied. Particularly when a piezoelectric field is generated in a light-emitting layer, the light-emitting layer exhibits an energy band slope. Therefore, the probability of recombination of electrons and holes is reduced; i.e., emission output is reduced.
In view of the foregoing, there has been developed a technique for relaxing stress applied to a light-emitting layer. For example, Patent Document 1 discloses a semiconductor laser device including an n-type guide layer and a p-type guide layer, wherein an InGaN layer more proximal to an active layer has a larger thickness (see paragraphs [0055] and [0056] and FIG. 10 of Patent Document 1). In the device, average In compositional proportion is regulated by changing the thicknesses of layers forming a superlattice structure.    Patent Document 1: Japanese Patent Application Laid-Open (kokai) No. 2010-177651
However, such a technique is still unsatisfactory for relaxing stress applied to a light-emitting layer. The lower the stress applied to a light-emitting layer, the more preferable the resultant device. This is because, when the stress applied to the light-emitting layer is reduced, the light-emitting device accordingly exhibits low energy band slope.